1. Field of the Invention
The present invention generally relates to the field of fabrication of integrated circuits, and, more particularly, to manufacturing processes involving the application of process liquids, such as plating solutions, onto the surface of a substrate.
2. Description of the Related Art
Integrated circuits comprise a large number of individual circuit elements, e.g., transistors, capacitors and resistors, formed on a substrate. These elements are connected internally by means of electrically conductive lines to form complex circuits such as memory devices, logic devices and microprocessors. In order to accommodate all the electrically conductive lines required to connect the circuit elements, in modem integrated circuits, the electrically conductive lines are arranged in a plurality of levels stacked on top of each other.
Frequently, the electrically conductive lines are formed by means of a so-called damascene process, wherein, on a semiconductor substrate, an interlayer dielectric is deposited in which vias and trenches are formed. These vias and trenches are then filled with metal, e.g., copper, to provide electrical contact between the circuit elements. To this end, a metal layer is deposited. In the following, the metal used for filling the vias and trenches will be denoted as “conductor metal.”
Frequently, electroplating is used for the deposition of the conductor metal layer. Electroplating is an electrochemical process which can be performed by means of specialized plating tools.
FIG. 1 shows a schematic cross-sectional view of a plating tool 100 according to the state of the art. The plating tool 100 comprises a process chamber 101 which is adapted to receive a plating solution 102. The process chamber 101 comprises an inner compartment 115 and an outer compartment 116 which are separated by a divider 117. The divider 117 and the outer compartment 116 run circularly around the inner compartment 115. An electrode 103, which is substantially comprised of the conductor metal, is provided within the inner compartment 115. The process chamber 101 further comprises a substrate holder 104 adapted to receive a substrate 105. A contact ring 106 provides electrical contact between the substrate 105 and the substrate holder 104. The electrode 103 and the substrate holder 104 are electrically connected to a power source 109, which is connected to a control unit 110.
Additionally, the plating tool 100 comprises a plating solution tank 111. A supply line 113 is adapted to supply the plating solution 102 from the plating solution tank 111 to the inner compartment 115. A pump 114 is configured to pump the plating solution 102 through the supply line 113. The pump 114 is connected to the control unit 110. A discharge line 112 is adapted to remove the plating solution 102 from the outer compartment 116 of the process chamber 101 and to supply it to the plating solution tank 111.
In operation, the control unit 110 activates the pump 114 in order to pump the plating solution 102 from the plating solution tank 102 to the inner compartment 115 of the process chamber 101, where it is conveyed to the substrate 105. The plating solution flows over the divider 117 from the inner compartment 115 into the outer compartment 116. From the outer compartment 116, the plating solution 102 flows back into the plating solution tank 111 via the discharge line 112.
The electrode 103 and the substrate 105 are in contact with the plating solution 102. The plating solution 102 comprises ions of the conductor metal. If the conductor metal comprises copper, the plating solution may be, e.g., an aqueous solution of copper sulfate comprising Cu2+ and SO42− ions. The control unit 110 controls the power source 109 to apply a current between the electrode 103 and the substrate holder 104. A polarity of this current, averaged over time, is such that the electrode 103 becomes an anode and the substrate 105 becomes a cathode. At the electrode 103, atoms of the conductor metal are positively ionized and change from a solid state in the electrode 103 into a dissolved state in the plating solution 102. At the substrate 105, positively charged ions of the conductor metal change from the dissolved state in the plating solution to the solid state and are deposited on the substrate 105. In the course of time, a metal layer 107 comprising the conductor metal is deposited on the surface of the substrate 105.
As a further step of the well-known damascene technique, a chemical mechanical polishing process is performed to remove excess metal deposited during the previous plating process to reliably fill the vias and trenches.
In the electroplating process described above, it is desirable to obtain a uniform thickness of the metal layer 107. The quality of the metal layer 107, however, may be adversely affected by gas bubbles in the plating solution 102. Such gas bubbles may be introduced into the plating solution by cascading flows such as the flow of the plating solution 102 over the divider 117 or tiny leaks in the tubes 112, 113 acting as Venturi vents. The bubbles may collect on the substrate 105, for example close to a center thereof, where they can impede the flow of the plating solution towards portions of the surface of the substrate 105 covered by the bubbles.
Thus, the bubbles adversely affect the transport of metal ions towards the portions of the substrate 105 covered by the bubbles. Therefore, a thickness of the deposited metal layer 107 on the portions of the substrate 105 covered by the bubbles can be significantly smaller than a thickness of the metal layer on other portions of the substrate 105. This may lead to an insufficient filling of vias and trenches. Insufficiently filled vias and trenches, in turn, may lead to malfunctions of integrated circuits formed on the substrate 105.
In view of the above problems, there is a need for an apparatus and a method allowing removal of bubbles from a process liquid. Moreover, there is a need for a plating tool wherein adverse effects of bubbles in the plating solution on the quality of a metal layer deposited by means of electroplating can be reduced.